Extruding diagonal rectangle from interior corner
Make 4 Loop Cuts (Crtl+R) at where it should intersect.





Select the 3 vertices IN THAT ORDER then press J to make a diagonal a line.





Repeat step 2, but with opposite vertices, to create both diagonal lines.





Select all 4 corners of the rectangle we have created.





Press E and then whatever axis they are facing, then bring them out until they are flush with the outside rim of the box.





Go into Edge Select Mode, then select these two edges, press X, then click Delete Edges .





Select all the points and press F and then press Ctrl+N, to Recalculate Normals.





DONE.


Let me know if thats not what you meant or if I messed up on a step!